[TowerTalk] Brake delay adjustment

[Tom Rauch]

> How do you keep the high charging current from damaging the 
> switch? Do you add enough series charging resistance, or does 
> the supply have enough ESR?

Let me clarify this a little, because it is useful in general 
applications.

If you have a switch or relay, be very careful adding a large 
capacitor after the switch or relay contacts. If you have any energy 
storage at all before the switch, or a supply capable of supplying 
any current, the switch contacts will arc on closure.

The amount of damage to the contacts will be related to the current 
and voltage differential across the contacts, the number of times 
the arc occurs, as well as the ability of the contact to withstand an 
arc. You might not always have an immediate failure, but you can 
be sure increasing the contact closure current by putting a large 
capacitive load on the contact will decrease the overall contact life.

If I were going to add a large electrolytic, and I wasn't sure of the 
contact rating or ESR and energy storage of the supply, I'd add a 
current limiting resistance of perhaps several dozen ohms in series 
with the charging path to the electrolytic.

As a second problem, a large capacitor will rob the solenoid of 
voltage (and current) at pull-in. Applying the voltage slowly will often 
make a relay or solenoid less reliable. 

For example, if I wanted to improve the brake release performance 
of the Ham-M series rotor (I'm supposed to do that soon, as part of 
a modification to the Hygain control boxes), I'd charge a large 
capacitor OVERVOLTAGE from the solenoid operating voltage. I'd 
hit the coil with twice or three times the operating voltage for a few 
seconds before the voltage settles down to the holding value.

If you tucked a capacitor across the solenoid line, you would 
change the slope of applied voltage so it was just the opposite of 
that required to insure reliable solenoid action.

Maybe the series resistance is already being applied properly. If 
not, it would be a good idea.


73, Tom W8JI
w8ji@contesting.com

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